Propionic acid (also called propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It can be used as solvent, as food preservative or in the herbicide manufacture. Propionic acid is also useful as an intermediate in the production of other chemicals, especially polymers. Cellulose-acetate-propionate is a useful thermoplastic. Vinyl propionate is also used as monomer in (co)polymers with e.g. ethylene, vinyl chloride and (meth)acrylic esters. In more specialized applications it is also used to make pesticides and pharmaceuticals. The esters of propionic acid have fruit-like odors and are sometimes used as solvents or artificial flavorings.
Processes for the production of propionic acid are known in the art. Currently, almost all propionic acid is produced by chemical synthesis from petroleum feedstocks.
US 2011/0124913 discloses a process for the industrial manufacture of propionic acid from glycerol. This makes it possible to obtain a bioresourced propionic acid from renewable resources. In this complicated chemical process containing several energy consuming isolation steps, glycerol is first dehydrogenated to acrolein, acrolein is oxidized to acrylic acid and acrylic acid is finally hydrogenated to propionic acid.
The propionic acid could also be produced by propionibacteria via the dicarboxylic acid pathway with acetic acid and succinic acid as byproducts, but low yield and productivity due to the inhibition of propionic acid on cell growth and propionic acid synthesis is a problem. Like most organic acid fermentations, the propionic acid fermentation is inhibited by acidic pHs and the major fermentation product, propionic acid. Furthermore, the fermentation is heterogenous, i.e. propionate is produced along with other by-products. This not only results in a low product yield but also renders product purification difficult and expensive.
To alleviate the inhibition of propionic acid on microbial growth and propionic acid synthesis, two approaches, extractive propionic acid fermentation and propionic acid production with propionic acid-tolerant bacteria obtained via adaptive evolution have been developed. Despite such advancements, current microbial propionic acid production cannot economically compete with petrochemical routes. Producing propionic acid from agricultural and industrial wastes may make microbial propionic acid production economically competitive. Glycerol is a main by-product of the biodiesel industry and could thus be a low-cost feedstock to produce propionic acid. While most studies on propionic acid production by Propionibacterium acidipropionici have focused on glucose and whey lactose, some studies have explored glycerol as the carbon source, and it was observed that glycerol might be advantageous since less acetic acid was produced during the consumption of glycerol (Zhu et al. Optimization and scale-up of propionic acid production by propionic acid-tolerant Propionibacterium acidipropionici with glycerol as the carbon source, Bioresource Technology 101 (2010), 8902-8906).
The rapidly expanding market for bioethanol and biodiesel is remarkably altering the cost and availability of glycerol. In general, approximately 10 pounds of crude glycerol are formed for every 100 pounds of biodiesel produced. Bioethanol process also generates glycerol up to 10% (w/w) of the total sugar consumed as a byproduct. Crude glycerol has thus been widely recognized as an attractive sustainable resource for chemical industries. Glycerol-based biorefinery is the microbial fermentation process using inexpensive and readily available glycerol as the raw material to produce fuels and chemicals. A major challenge in fermentation of the low-grade crude glycerol is to obtain microbial strains tolerant to undesirable inhibitory components such as salts and organic solvents that present in crude glycerol. There have been several attempts to explore anaerobic microbial assimilation of glycerol using reconstructed microbial systems via microbial screening and metabolic pathway engineering. As a result, fuels as well as some high-value products were found to be produced by microbial fermentation of glycerol (Choi, W. J., Glycerol-Based Biorefinery for Fuels and Chemicals, Recent Pat Biotechnol. 2008; 2(3):173-80).
Glycerol (or glycerin) is the principal component of triglycerides and it is clear, odorless and viscous liquid. It is found in animal fats, vegetable oils or petrochemical feedstocks, and it is derived from soap or biodiesel production. Choi (2008, supra) describes microbial metabolic pathways of glycerol for fuels and chemicals, and different glycerol-based fermentative products, such as ethanol, biogas and organic acids.
Zhu et al. (2010, supra) describe propionic acid production by propionic acid-tolerant Propionibacterium acidipropionici with glycerol as sole carbon source in batch cultures and then scaled-up production in a 10 m3 fermentor using the optimized conditions.
Zhang and Yang (Process Biochemistry 44 (2009) 1346-1351) describe a process for propionic acid production from glycerol by using metabolically engineered Propionibacterium acidipropionici in free-cell fermentation.
Boyaval, P. et al (Enzyme Microb Technol., 1994, vol. 16, 883-886) describe fermentation of glycerol by propionic acid bacteria to obtain propionic acid with no acetic acid. The method comprises subjecting the medium to ultrafiltration.
Extractive fermentation is generally a process for producing a variety of chemical products by fermentation in which the product is removed from the fermentation medium as it is formed by liquid-liquid extraction using an extractant which is immiscible with water.
Yang and Lewis (A Novel Extractive Fermentation Process for Propionic Acid Production from Whey Lactose, Biotechnol. Prog, 1992, 8, 104-110) describe an extractive fermentation process to produce propionate from lactose. Extractive fermentation removes the inhibitory acidic product from the reactor and therefore provides better pH control on the reactor and results in higher reaction rates. Alamine 336/2-octanol mixture was used as the extractant in an extractive fermentation process for propionate production from whey lactose. In lactose fermentation the main by-products are acetic acid and carbon dioxide.
There is a need for novel and efficient methods for purifying and recovering propionic acid from fermentations wherein glycerol is used as the starting material. The specific conditions, such as different by-products than in e.g. lactose fermentation, make it challenging.